An active electronically scanned array (ESA), also known as an active electronically steered antenna, is a type of phased array whose transmitter and receiver functions are composed of numerous small solid-state transmit/receive modules. Active ESAs aim their beam by emitting separate radio waves from each module that interfere constructively at certain angles in front of the antenna. Another type of ESA, referred to as the passive ESA, is a phased array which has a central radio frequency source (such as a magnetron, a klystron, a travelling wave tube or solid state amplifier), sending energy into (usually digitally controlled) phase shift modules, which then send energy into the various emitting elements in the front of the antenna.
An ESA will be confined to a limited area set by its application and platform installation. The pitch between adjacent elements is about half of the wavelength of the carrier and as frequency increases, the pitch between adjacent elements decreases. The efficiency of the transmitter typically degrades over the frequency. Therein lies a need to provide high efficiency transmitters for ESAs.